1. Field of the Invention
The present invention relates to a line illumination device which is installed in an image reading apparatus, including a facsimile machine, a copy machine and a hand scanner etc.
2. Description of the Related Art
In apparatuses such as facsimile machines, copy machines and hand scanners, etc., an image reading apparatus such as an image scanner has been adopted as a device for reading images such as text. For the types of such image sensors, there are various types, for example: a reducing type, a close contacting type, a completely close contacting type, and so on. Among of those, the image sensor of the close contacting type is constructed with an illumination device, an equimultiple image forming optical device and a sensor, and so on. And, the close contacting type image sensor, having a short optical path thereby contributing to the small size of the apparatus, generally, necessitates no troublesome optical adjustment to be installed into the apparatus, in comparison with the image sensor of such the reducing type. Therefore, it recently has been widely used in place of the reducing type of image sensor for the advantages mentioned above.
The illumination device for use in the close contacting type image sensor must irradiate light onto the surface of the text much more than the illumination that is sufficient for the sensor to perform a read or scan operation. And the area, on which the illumination device illuminates, is line-like, and is relatively long in a main scanning direction (a longitudinal direction), while it may be very narrow in a sub-scanning direction which is perpendicular to the main scanning direction. For instance, the device used in a facsimile machine of a so-called A4 size necessitates a length of 216 mm in the longitudinal direction. And, if the illuminance on the text surface fluctuates in the longitudinal direction, this results in an error in the reading operation of the apparatus. Therefore, it is preferable for the illuminance to be uniform as far as it is possible.
For such an illumination device, an LED type illumination device has been conventionally known, in which a few tens (for example, 30) of LEDs (Light Emitting Diodes) are aligned in a line on a printed wiring board and installed by wire bonding or soldering thereon. In the close contacting type image sensor comprising such an illumination device, light is emitted from the illumination device and is irradiated onto the text surface to be read through a cover glass on which the text is mounted or laid, and an image on the text surface is read by a photoelectric conversion element through a rod lens array.
For such a conventional device or apparatus in which many LEDs are aligned, however, only a portion of the light radiating onto a fine line on the text to be read is actually effective for the reading and the remaining portion thereof is not, i.e., is only wasted. Additionally, there is also known a method, in which the illumination device is positioned as near as possible to the text to be read so as to increase the illuminance, therefore decreasing the number of LEDs that must be installed or provided. However, a greatly fluctuating illuminance results with this method. Namely, with such a construction of the light source or illumination device, there is basically caused a fluctuation in the quantity of light, depending on the pitch of the installed LEDs, more or less. Further, there is also caused a fluctuation in the quantity of light due to differences in characteristics amongst the aligned LEDs themselves, the differences being caused in the production process thereof. Consequently, the fewer LEDs which are installed in one illustration device, the greater the fluctuation in the quantity of light or in the illuminance on the reading line on the text to be read.
Therefore, the same inventors of the present invention have proposed a technology, in Japanese Laid-Open Patent No. 6-148435 (1994) and Japanese Laid-Open Patent No. 7-14414 (1995), in which light emitting elements (LEDs) are provided at both ends of a rod-like transparent member, or a light scattering surface is made on a portion of the surface of the transparent member.
However, with the line illumination device disclosed in the references mentioned above, since the light emitting elements are provided at both end portions of the rod-like transparent member, it is therefore possible to remove one of them. Next, the same inventors of the present invention have also proposed a line illumination device, in Japanese Laid-Open Patent No. 8-163320 (1996) and Japanese Laid-Open Patent No. 8-172512 (1996), in which the light emitting element is provided only at one end of the rod-like transparent member. With the line illumination device disclosed in these official publications, as well as achieving a decrease in the cost thereof by positioning the light emitting element only at one end of the rod-like transparent member, a light scattering pattern is formed on a surface of the rod-like transparent member in such a shape that it gradually increases in area from the one end on which the light is introduced from the light emitting element up to or towards the other end, thereby obtaining a uniform illuminance along the longitudinal direction thereof.
As described in the above, in the line illumination device in which the light emitting element is provided only at the one end portion of the rod-like transparent member, mirror surfaces are made not only at the one end surface of the rod-like transparent member, but also at the other end surface thereof. On the other end surface, which is further provided with a material having a high reflection factor on the outer side of the mirror surface of the other end surface, incident light, which is introduced at the one end portion of the rod-like transparent member and which reaches to the other end surface without being scattered by the scattering pattern, is reflected thereon and turns back to the other end side passing within the rod-like transparent member. And, by repeating this process many times, all the incident light can be utilized as irradiating light from the light scattering pattern.
It can be made by a mold injection if it has the mirror surface at the one end surface thereof. However, if it has mirror surfaces at both ends, a polishing process must be made after a cutting process is performed at the other end portion (a gate portion) by a jig such as a nipper and so on, thereby causing an increase in the number of production steps and in production costs thereof.